1. Field of the Invention
This invention relates generally to the field of solid freeform fabrication (xe2x80x9cSFFxe2x80x9d) of parts and more specifically to the powder blend for use in the selective laser sintering process utilizing a steel alloy and the method of forming three-dimensional parts employing that powder blend.
2. Description of the Relevant Art
SFF generally refers to the manufacture of articles in a layer-wise or additive fashion directly from computer-aided-design (CAD) databases in an automated fashion, as opposed to conventional machining of prototype articles from engineering drawings in subtractive processes. SFF has, in recent years, made substantial improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. As a result, the time required to produce prototype parts from engineering designs has reduced from several weeks, using conventional machinery and subtractive processes, to a matter of hours.
One example of an SFF technology is the selective laser sintering process practiced by systems available from 3D Systems, Inc. of Valencia, Calif. According to this technology, articles are produced in layer-wise fashion from a laser-fusible powder that is dispensed one layer at a time. The powder is fused, or sintered, by the application of laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. After the fusing of powder in each layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete. Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538 and 5,017,753, both assigned to Board of Regents,
The University of Texas System, and in U.S. Pat. No. 4,247,508, to Housholder. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including nylons, polystyrenes, and composite materials such as polymer coated metals and ceramics. Examples of composite powder materials are described in U.S. Pat. Nos. 4,944,817; 5,076,869; and 5,296,062, all assigned to Board of Regents, The University of Texas System, and incorporated herein by reference in pertinent part.
A related SFF technology, referred to as 3-Dimensional (3D) Printing, is described in U.S. Pat. Nos. 5,340,656 and 5,387,380. From a computer (CAD) model of the desired part, a slicing algorithm draws detailed information for every layer. Each layer begins with a thin distribution of powder spread over the surface of a powder bed. Using a technology similar to ink-jet printing, a binder material selectively joins particles where the object is to be formed. A piston that supports the powder bed and the part-in-progress lowers so that the next powder layer can be spread and selectively joined. This layer-by-layer process repeats until the part is completed. Following a heat treatment, unbound powder is removed, leaving the fabricated part.
As SFF technology has evolved, it has increasingly been used not only to make prototype parts but also to make final useful parts as well as tools or molds that can be used to make multiple parts. It is becoming more common to fabricate such parts, tools, or molds with an xe2x80x9cindirectxe2x80x9d process that uses a powder of metal and/or ceramic particles either coated by or blended with a polymer. The powder is used in the selective laser sintering process to fabricate a xe2x80x9cgreenxe2x80x9d article that binds the particles to one another. The green article is then heated to a temperature above the decomposition temperature of the polymer, which both drives off the polymer and also binds the metal and/or ceramic substrate particles to one another to form an intermediate porous article. The porous article can then be infiltrated with another material, such as a lower melting temperature metal to give a fully dense article with desirable properties. The green article can also be fabricated with 3D printing.
Some examples of the use of these approaches for functional applications are described, for example, in U.S. Pat. Nos. 5,433,280: 5,544,550 and 5,839,329 to Smith et al. These describe the use of selective laser sintering a tungsten carbide-polymer composite powder to generate xe2x80x9cgreenxe2x80x9d drill bit which is then infiltrated in a furnace cycle with a copper alloy to generate a fully functional drill bit for down hole oil exploration. U.S. Pat. No. 4,554,218 describes the use of a powder mixture having a first metal and a second metal, such as A6 tool steel, and a fugitive binder that is placed in a mold, cured to a green part and then infiltrated with a third metal, preferably a copper or copper-containing alloy, to form an infiltrated, molded metal composite article. Another commercial application of these indirect approaches is a product called ProMetal by ExtrudeHone. Utilizing the 3D Printing technology described above, ProMetal builds metal components by selectively binding metal powder layer by layer. The finished structural skeleton is then sintered and infiltrated with bronze to produce a finished part that is 60% steel and 40% bronze and is used for injection molding tools or final metal parts. Another commercial example is 3D Systems"" ST-100 system, which uses selective laser sintering of a steel polymer composite powder to generate a green article. The green article or part is subsequently put through a furnace cycle that removes the polymer binder and infiltrates the metal skeleton with bronze to create a functional fully dense article that can also be used for injection mold tools or final parts.
As is well known in the art, the structural strength of the green article is an important factor in its utility, since weak green articles cannot be safely handled during subsequent operations. Another important factor in the quality of a prototype article is its dimensional accuracy relative to the design dimensions. However, these factors of part strength and dimensional accuracy are generally opposed to one another, since the densification of the powder that occurs in the sintering of the post-process anneal also causes shrinkage of the article. The polymer content of a metal and/or ceramic composite powder described above could be increased in order to provide higher green part strength, but the shrinkage of the part in post-process anneal would increase accordingly. As a result, compromises between article strength and dimensional stability must be made in the design of a composite powder system using a polymeric binder.
Some drawbacks of conventional composite powders incorporating thermoplastic polymer binders have been observed. In the post-process anneal of green articles using such binders, creep deformation has been observed as the article is heated to a temperature above the glass transition temperature of the polymer binder, but below the decomposition temperature at which the binder is released. The viscosity of the polymer decreases to such an extent that the metal or ceramic substrate particles slide past one another under the force of gravity. Not only do the dimensions of the article change as a result of this creep deformation, but also this dimensional change is not uniform. Taller features deform by a larger extent than do shorter features. This non-uniformity in deformation precludes the use of a constant shrinkage correction factor in the selective laser sintering fabrication of the green part, further exacerbating the difficulty of achieving dimensionally accurate articles of high density and strength.
Creep deformation has been observed to deform not only the height but also the shape of vertical features, such as sidewalls. For example, vertical walls of mold cavities formed by selective laser sintering of polymer-coated metal powders, and having a thickness of 0.75 inches and a height of 1.5 inches, have been observed to bow outwardly as a result of creep deformation. The dimensional accuracy of the infiltrated final part is, of course, severely compromised by such deformation.
To address this tendency of creep deformation, another prior art technique was developed that combined the use of a thermoplastic binder with a thermoset binder. This is described in U.S. Pat. No. 5,749,041. In this approach a xe2x80x9cgreenxe2x80x9d part is formed by the selective laser sintering of a metal-polymer composite powder, in which the polymer binder is a thermoplastic polymer. Following its fabrication, the green article is infiltrated with a thermosetting material prior to heating the part. The thermosetting material may be an aqueous emulsion of a cross-linkable polymer with a cross-linking agent, or may instead be an aqueous emulsion of only the cross-linking agent. In the first case, the cross-linking agent reacts with the cross-linkable polymer in the infiltrant to form a rigid skeleton for the green article; in the second case, the cross-linking agent reacts with the polymer binder of the green article to form the rigid skeleton. Following the formation of the rigid skeleton, the article may be heated to decompose the polymer and sinter the metal substrate particles, followed by infiltration with a metal for added strength. This prior art approach provided a solution to the creep deformation problem, but added significant time to the post processing of the part to dry out the article after the aqueous infiltration step.
Another approach used commercially to avoid the aforementioned drying step incorporates both a thermoplastic and thermoset binder in the formulation of the metal-polymer composite article. In one successful version a phenolic type thermoset is combined with a wax binder to give a system that provides adequate initial green strength and a more rigid skeleton for the green article. The green strength of this system though, while improved, still suffers from unacceptable failure rates due to breakage of green parts in handling. Thus the search for stronger green part systems has continued.
The trend has been to use more polymer binder materials as one approach to achieve higher green strength parts. However, as the amount and complexity of binders used in these metal and/or ceramic polymer composite powders has increased, it has been increasingly difficult to removing all of the polymer system binders during the decomposition and burn-out phase. The decomposition of the polymer into smaller fragments should be complete enough to ensure that the bulk of the hydrocarbon fragments can escape the article skeleton before the infiltrating metal (copper or bronze, for example) enters the skeleton. If all of the hydrocarbon fragments do not escape, the interconnectivity of pores in the resulting metal part is decreased and outgassing is hampered as the interpassages become blocked by trapped hydrocarbon fragments leading to a phenomena of blistering on the surface and potential delamination of the final article. In some systems the presence of too much residual carbon can also impede the infiltration process. The presence of a reducing atmosphere, such as hydrogen or forming gas helps the polymer degradation greatly, but is a more expensive alternate than a non-reducing gaseous atmosphere.
Accordingly, there is a need to provide an improved powder blend for use in conjunction with laser sintering and an infiltration process to achieve finished articles possessing high strength, enhanced toughness (resistance to crack growth), increased surface hardness, increased corrosion resistance or stainless property and improved surface finish with less distortion and shrinkage during heat treatment and infiltration.
It is an aspect of the present invention to provide a method of fabricating high density and high strength articles and tooling via SFF techniques from powder blend and a powder blend comprising at least a steel alloy selected from the group consisting of mild steel, carbon steel and stainless steel, a polymeric binder and a high melting temperature fine particulate metallic, intermetallic or ceramic with improved initial green strengths.
It is another aspect of the present invention that a metal powder blend is employed which provides improved part building in a laser sintering process.
It is a still another aspect of the present invention to provide such a powder blend comprising at least a steel alloy selected from the group consisting of mild steel, carbon steel and stainless steel, a polymeric binder and a high melting temperature fine particulate metallic, intermetallic or ceramic, and a method using such a powder blend in a laser sintering SFF technique that improves dimensional accuracy.
It is a further aspect of the invention to provide such a method for use in a laser sintering SFF technique while avoiding blistering phenomena even in nitrogen atmospheres.
It is a feature of the present invention that the base alloy is mild steel that yields low shrinkage during infiltration with a preferred metal infiltrant.
It is another feature of the present invention that a preferred infiltrant is copper and/or a copper containing alloy.
It is still another feature of the present invention that the material composition is blended for an extended period of time to break up agglomerations of a high melting temperature fine particulate metallic, intermetallic or ceramic material.
It is a further feature of the present invention that the infiltration occurs in a gas atmosphere.
It is yet another feature of the present invention that the polymeric binder is a co-polymer of nylon 6 and nylon 12.
It is still another feature of the present invention that the metal powder composition is able to employ a lower percentage of binder material to increase the green strength of the part formed from the laser sintering process.
It is an advantage of the present invention that the powder blend and method using such powder blend produce high quality, fine-featured green parts with minimal curl.
It is an another advantage of the present invention that the powder blend and method using such powder blend produce parts after infiltration with excellent surface finish and high hardness.
It is a further advantage of the present invention that the powder blend and method using such powder blend produce parts after infiltration having a desirable balance of toughness, strength and wear resistance.
It is yet another advantage of the present invention that the powder blend and method using such powder blend produce parts with excellent thermal conductivity after infiltration with a copper containing alloy.
It is a further advantage of the present invention that the metal powder composition flows without caking as it is distributed across the part bed of the laser sintering system.
It is yet a further advantage of the present invention that the laser sintered part obtained using the metal powder composition of the present invention has a higher green strength than prior metal compositions despite not employing an increased binder composition.
It is still a further advantage of the present invention that the laser sintered part obtained using the metal powder composition of the present invention is corrosion resistant or possesses stainless property.
The invention may be incorporated into a method of fabricating an article, such as a prototype part or a tooling for injection molding, using selective laser sintering. According to the present invention, the selective laser sintering of a powder blend comprising at least a steel alloy selected from the group consisting of mild steel, carbon steel and stainless steel, a polymeric binder, and a high melting temperature fine particulate metallic, intermetallic or ceramic forms a xe2x80x9cgreenxe2x80x9d part. After removal of unfused material from the green part, it is placed in an oven or furnace preferably in a non-reducing atmosphere such as, for example, nitrogen, argon or a nitrogen-argon blend for subsequent heat treatment to decompose and drive off the binder and sinter the metal substrate particles prior to infiltration by a metal with a lower melting point. During the critical step of infiltrating the green part, the green part is packed in a fine grit alumina in a non-reducing gas atmosphere that creates the conditions necessary to provide sufficient surface area to absorb the outgassing of the binder material and prevent redeposit on the part. A preferred steel alloy is a mild steel alloy and a preferred high melting temperature fine particulate is tungsten carbide.
These and other aspects, features and advantages are obtained in the present invention wherein the powder blend for use in a laser sintering process comprises a steel alloy selected from the group consisting of mild steel, carbon steel and stainless steel, a polymeric binder, and a high melting temperature fine particulate metallic, intermetallic or ceramic and the powder blend is employed in a method to form a tough, strong and wear resistant product comprising the steps of:
a. mixing together a powder blend comprising a steel alloy selected from the group consisting of mild steel, carbon steel and stainless steel, a high melting temperature fine particulate metallic, intermetallic or ceramic and a polymeric binder;
b. applying a layer of the powder blend to a working surface in a laser sintering system;
c. exposing the layer of the powder blend to heat energy to fuse together the mild steel alloy and tungsten carbide by the melting and subsequent rehardening of the binder material;
d. applying a new layer of powder blend and exposing the new layer of powder blend in sequential fashion repeatedly until a three-dimensional green metal part is formed; and
e. infiltrating a green metal part with metal infiltrant in a non-reducing gas atmosphere at an effective temperature for an effective time period in an oven.